A bandgap reference circuit is an important module for most electronic circuits and systems. Such circuit may provide reference voltages and/or reference currents that must be independent of process, supply and temperature. In some cases, the reference currents may be inversely proportional to the value of a reference resistor.
Conventional bandgap reference circuits provide reference voltages and/or reference currents with temperature variations that are inadequate for high-precision applications. As a result, high-order temperature curvature compensation circuits are added to solve this issue.
Conventional bandgap reference circuits that provide a voltage reference are referred to as bandgap voltage reference circuits. Typically, no load currents are required from the output of a bandgap voltage reference circuit. As a result, a high impedance at the output of the bandgap voltage reference circuit is acceptable. In contrast, a low output impedance is needed when high load currents or high leakage currents are present at the output; otherwise, the output reference voltage will vary as a function of the output load current. This can be solved by using an error amplifier output as the bandgap reference circuit output. While this is possible for a traditional bandgap reference circuit, the output voltage of which is equal to (or near) the silicon bandgap voltage (1.22 V at 0K), it is not easy for a scalable bandgap reference circuit, the output voltage of which can be of any value above or below the silicon bandgap voltage, irrespective of the silicon bandgap voltage. High-precision applications that need load or leakage currents from a scalable bandgap reference circuit require both high-order temperature curvature compensation and low output impedance. The silicon bandgap voltage is a reference voltage commonly used in the art. This may be referred to as a “golden voltage” in this description.
FIG. 1 shows a prior art bandgap voltage reference circuit, wherein the output reference voltage is VREF. The circuit comprises a conventional bandgap voltage reference circuit made of the two bipolar junction transistors Q1 (101) and Q2 (102), the three resistors R1 (103) and R2 (i.e., (104) and (105)), an operational amplifier (106), the MOSFET current sources M1 (107) and M2 (108), and the output branch made of the MOSFET current source M3 (109) and resistor R3 (110). The currents flowing in the bipolar transistors Q1 (101) and Q2 (102) are proportional to the absolute temperature (PTAT), while the currents flowing in the resistors R2 (e.g. (104) and (105)) are complementary to the absolute temperature (CTAT). As a result, the currents in the MOSFET current sources M1 (107), M2 (108) and M3 (109) are almost independent of the absolute temperature. However, due to the non-linear temperature dependence of the CTAT currents originating from the bipolar junction forward voltage, the reference voltage VREF shows temperature dependence that is usually not acceptable in high-precision applications. The circuit uses the bipolar junction transistor Q3 (111), the resistors RNL (i.e. (112) and (113)), and the MOSFET current source M4 (114) to subtract the non-linear temperature dependence of the CTAT currents to yield an almost constant reference with respect to temperature variations.
FIG. 2 shows a prior art bandgap voltage reference circuit where the output reference voltage is VREF. The circuit comprises a conventional bandgap voltage reference circuit made of the two bipolar junction transistors Q1 (201) and Q2 (202), the three resistors R1 (203), and R2 (i.e., (204) and (205)), an operational amplifier with low output impedance (206), and a transconductance amplifier (207). With the voltage reference VREF being the output of the operational amplifier, a low output impedance can be guaranteed at the output of the bandgap reference circuit. Accordingly, it can provide an output current or support a high leakage current. The transconductance amplifier must have a linear relationship between its input voltage and output current that has very small dependence on temperature.
FIG. 1 and FIG. 2 discuss prior art bandgap voltage reference circuits, where either high-order temperature curvature compensation or low output impedance is used, but not both. Such bandgap voltage reference circuits cannot be used in high-precision applications when the bandgap voltage reference circuit needs to support a load or leakage output current.